UNIQUE KEY | UNIQUE Constraint in SQL - sql - sql tutorial - learn sql




  • The UNIQUE constraint ensures that all values in a column are different.
  • Both the UNIQUE and PRIMARY KEY constraints provide a guarantee for uniqueness for a column or set of columns.
  • A PRIMARY KEY constraint automatically has a UNIQUE constraint.
  • However, you can have many UNIQUE constraints per table, but only one PRIMARY KEY constraint per table.

SQL UNIQUE Constraint on CREATE TABLE

  • The following SQL creates a UNIQUE constraint on the "ID" column when the "Persons" table is created:

SQL Server / Oracle / MS Access:

CREATE TABLE Persons (
    ID int NOT NULL UNIQUE,
    LastName varchar(255) NOT NULL,
    FirstName varchar(255),
    Age int
);

MySQL:

CREATE TABLE Persons (
    ID int NOT NULL,
    LastName varchar(255) NOT NULL,
    FirstName varchar(255),
    Age int,
    UNIQUE (ID)
);
  • To name a UNIQUE constraint, and to define a UNIQUE constraint on multiple columns, use the following SQL syntax:

MySQL / SQL Server / Oracle / MS Access:

CREATE TABLE Persons (
    ID int NOT NULL,
    LastName varchar(255) NOT NULL,
    FirstName varchar(255),
    Age int,
    CONSTRAINT UC_Person UNIQUE (ID,LastName)
);
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SQL UNIQUE Constraint on ALTER TABLE

  • To create a UNIQUE constraint on the "ID" column when the table is already created, use the following SQL:

MySQL / SQL Server / Oracle / MS Access:

ALTER TABLE Persons
ADD UNIQUE (ID);
  • To name a UNIQUE constraint, and to define a UNIQUE constraint on multiple columns, use the following SQL syntax:

MySQL / SQL Server / Oracle / MS Access:

ALTER TABLE Persons
ADD CONSTRAINT UC_Person UNIQUE (ID,LastName);
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DROP a UNIQUE Constraint

  • To drop a UNIQUE constraint, use the following SQL:

MySQL:

ALTER TABLE Persons
DROP INDEX UC_Person;

SQL Server / Oracle / MS Access:

ALTER TABLE Persons
DROP CONSTRAINT UC_Person;

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